1. Field of the Invention
The present invention relates to IC output driver circuits, and more particularly to a method and apparatus for providing precise on-chip bus pull-up impedance for N-channel open drain output drivers where the impedance presented to the bus is insensitive to fluctuations in temperature and voltage, and variations due to fabrication.
2. Description of the Related Art
In earlier integrated circuit (IC) designs, CMOS output drivers were configured as push-pull elements. Consequently, the noise seen on an output bus fluctuated significantly in response to various factors, including circuit temperature, supply voltage, fabrication process differences, the number of devices on the bus, etc. In more recent years, as technological advances have resulted in the scaling of device size and voltage levels, designers have been forced to aggressively address noise problems on external buses in order to maximize the operating speed of circuits within a system. A bus typically includes one or more signal lines collectively routed on a system board or the like, where each signal line can be modeled as a transmission line subject to noise (e.g., reflections, cross-talk, etc.).
One aspect of more recent output driver solutions has been a move in the industry from push-pull output configurations to differential receiver configurations. In a differential receiver configuration, one side of a differential receiver is supplied with a reference voltage and the other side is driven by an open drain N-channel device. The open drain N-channel devices are provided on-chip and bus pull-up terminations are generally provided externally, typically on a system motherboard or the like. Providing pull-up terminations on a motherboard grants system designers a level of flexibility to address bus noise problems that has heretofore been unavailable.
The aforementioned types of output drivers have become prevalent within the industry. One particular example of this prevalence is exhibited by the Pentium® II x86 microprocessor, a product of the Intel® Corporation. The Pentium II employs open drain N-channel output devices to drive a 1.5 Volt (V) bus having a 1.0 V reference threshold. Motherboards for this processor generally employ 56 ohm pull-up terminations. Although no particular pull down impedance has been specified, open drain output drivers have been used to comply with bus switching and timing specifications. Without compensating for process, voltage, and temperature variations, however, the channel resistance of an open drain N-channel output driver can vary anywhere from approximately 4 to 80 ohms. And since microprocessor designers can only anticipate acceptable limits for process, voltage, and temperature fluctuations, they have been compelled in Pentium II motherboard-compatible designs to add slew rate controls on the order of 2–3 nanoseconds (ns) to output signal edges to reduce noise on output buses.
With the Pentium III®, Intel introduced a means whereby designers are provided with a reference impedance that can be used to set the impedance of the output drivers on the bus. A pin on the processor package, referred to as NCHCTRL, is connected to the bus voltage, referred to as VTT, through a precision 14 ohm resistor, with a maximum specified resistance of 16 ohms. The precision resistor is external to the microprocessor chip and is therefore independent of the temperature and voltage variations seen by output drivers on the chip. In addition, pull-up terminations for compatible configurations are to be provided on-chip rather than on the motherboard of a system. And another pin, RTT, is provided, to which a precision resistor, R, is to be connected between the pin and ground. The impedance across the precision resistor indicates the desired impedance for all pull-up terminations. Hence, a system designer is able to set the bus pull-up impedance for all of the signals on a part through one external resistor. By specification, the resistor can range from 40 to 130 ohms, thus enabling system designers to adjust the pull-up terminations on N-channel open drain buses to compensate for noise or loading.